Republic of ChinaMercury (Hg2+) and silver (Ag+) ion-selective PVC membrane electrodes based on 1,4-dithia-I 2-crown-4 and 1,4-dithia-l5-crown-5 as neutral carriers were successfully developed. Both electrodes exhibited good linear responses of 30 and 40 mV decade-1 for Hg2+ and Ag+/ respectively, within the concentration ranges 10-2-10-6 M Hg(N03)2 and 10-1-10-6 M AgN03. Some other crown ethers and cryptands were also investigated as neutral carriers for both ions. Both the mercury and silver electrodes exhibited comparatively good selectivities for mercury(l1) and silvertl) ions in comparison with alkali metal, alkaline earth metal and some heavy metal ions. These crown ether ion-selective electrodes are suitable for use with aqueous solutions at pH 3 2. They were applied as sensors in titrations of Br-and CIwith Ag+ and of Hg2+ with 1-and Cr2072-and in the determination of the solubility products of AgCl in aqueous solutions.

117, Republic of ChinaCrown ethers, such as 15-crown-5, 12-crown-4 and dibenzo-18-crown-6, were used to extract individual rare earth ions from aqueous solutions containing picrate into nitrobenzene solution. The rare earth metal ion europium(II1) is extracted as a 2 : 1 crownion sandwich complex with 12-crown-4, but as a 1 : 1 complex with both 15-crown-5 and dibenzo-18-crown-6. From studies of picric acid concentration effects on complexation, the extracted species of Eu (NO,), with 15-crown-5 and dibenzo-18-crown-6 are E u [ ( 15-crown-5)-(picrate),(NO,)] and Eu[(dibenzo-l8-crown-6) (picrate) ,(NO,)], respectively, but Eu[( 12-crown-4), (picrate),] is formed with 12-crown-4. The extraction of rare earth ions showed that Tb3+, Eu3+, Gd3+, Nd3+ and Yb3+ can be easily extracted using 15-crown-5; however, the extraction of Ce4+, Sm3+, Dy3+ and Lu3+ is more difficult. Keywords Rare earth metals ; solvent extraction; crown ethers I II 111 Experimental ReagentsCrown ethers, 15crown-5 and 12-crown-4, were obtained from E. Merck. Dibenzo-18-crown-6 was synthesised All reagents except the following were of analytical-reagent grade.

Using high-performance liquid chromatography it has been possible to separate 14 lanthanide elements in less than 40 min by elution with an ahydroxyisobutyric acid concentration gradient. LiChrosorb KAT cation exchanger (10 pm) is used as the packing material and the eluted lanthanides are detected with an ultraviolet detector after complexation with Arsenazo 111. The detection limit is about 0.1 pg.Determinations of the rare-earth composition in monazite sand and .rareearth impurities in high-purity lanthanum(III), cerium(IV), samarium(II1) and yttrium(II1) oxides were carried out. The proposed method is more convenient than others, and the separation of major components is not necessary in analyses of high-purity rare earths; the elution time is only about 1 h.

Granular entrapped adsorbents of crown etherphosphotungstic acid (PW) and crown etherphosphomolybdic acid (PMo) in polyacrylamide were prepared by the solgel method and used for the adsorption of various heavy metals ions such as Ag+, Pb2+, Hg2+, Cd2+, Fe3+ and Zn2+. Dibenzo-18-crown-6 -PW (or -PMo)/polyacrylamide adsorbent gave high adsorption and a high selectivity for large metal ions such as Ag+, Pb2+ and Hg2+, but not for smaller ions. In contrast, small heavy metal ions such as Cu2+, Fe3+ and Zn2+, but not larger ions, were strongly adsorbed by 12-crown-4 -PW (or -PMo)/polyacrylamide adsorbent. The effects of pH and solvents on the adsorption were investigated. A dibenzo-18-crown-6 -PMo adsorbent film was also prepared and successfully applied to the selective adsorption of Ag+ ions from an aqueous solution containing various metal ions, and the Ag+ ions on the film could be determined quantitatively by X-ray fluorescence analysis.

Crown etherphosphotungstic acid (PW) and crown etherphosphomolybdic acid (PMo) precipitates, and also the granular entrapped crown ether -PMo (or PW) in polyacrylamide, were prepared and applied as adsorbents for rare earth metal ions. Adsorbents containing 15-crown-5, such as 15-crown-5 -PMo, were better adsorbents than the other crown ether precipitates. The adsorption capacity of 15-crown-5 -PMo for Eu3+ was determined and corresponded to about 0.166 mmol of Eu3+ per gram of the adsorbent. The effects of pH and metal ion concentration on adsorption were also investigated. Crown ether -PMo (or PW) precipitates underwent hydrolysis at pH 2 1, but the granular entrapped crown ether -PMo (or PW)polyacrylamide adsorbents were not hydrolysed at pH 2 1. The adsorption of individual rare earth ions with 15-crown-5 -PMopolyacrylamide showed that Tb3+, Nd3+, Eu3+ and Gd3+ were readily adsorbed, but adsorption was difficult for Ce4+, Sm3+ and Dy3+.